Many of today's field effect transistor circuits use current mirrors each of which comprise essentially a field effect transistor with the source connected to a power supply and the gate terminal connected to a potential which is of sufficient magnitude and correct polarity to allow the desired current level to flow through the transistor. If the gate terminal potential is held fixed and the power supply potential connected to the source terminal of the transistor varies, the gate to source potential of the transistor changes. This changes the current flowing in the mirror. Many applications require that the current flow be at a preselected level, and remain at or very near that level independent of power supply, temperature, and most processing changes. Some of the available current sources do not adequately meet these requirements, or use one or more off chip components.
It is desirable to have a transistor type current source with the gate terminal coupled to voltage generator circuitry which causes a relatively constant current level to flow through the transistor even with power supply, temperature, and processing variations, with all components on chip. Many applications also require the absence of switching noise, precluding switched capacitor approaches.